1. Field of the Invention
The present invention relates to a display device and electronic equipment, and more particularly to a flat panel display device and electronic equipment having the same in which pixels, each incorporating an electro-optical element, are disposed in a matrix form.
2. Description of the Related Art
In the field of image display device, flat panel display devices having pixels (pixel circuits), each incorporating an electro-optical element, disposed in a matrix form, are rapidly becoming widespread. Among flat panel display devices, the development and commercialization of organic EL display devices using organic EL (Electro Luminescence) elements have been continuing at a steady pace. An organic EL element is a type of current-driven electro-optical element whose light emission brightness changes according to the current flowing through the element. This type of element relies on the phenomenon that an organic thin film emits light when applied with an electric field.
An organic EL display device has the following features. That is, it is low in power consumption because organic EL elements can be driven by a voltage of 10V or less. Besides, organic EL elements are self-luminous. Therefore, an organic EL display device offers higher image visibility as compared to a liquid crystal display device designed to display an image by controlling the light intensity from the light source (backlight) for each of the pixels containing liquid crystal cells. Further, an organic EL display device desires no lighting members such as backlight as desired for a liquid crystal display device, thus making it easier to reduce weight and thickness. Still further, organic EL elements are extremely fast in response speed or several μ seconds or so. This provides a moving image free from afterimage.
An organic EL display device can be either simple (passive)-matrix or active-matrix driven as with a liquid crystal display device. It should be noted, however, that a simple matrix display device has some problems although simple in construction. Such problems include difficulty in implementing a large high-definition display device because the light emission period of the electro-optical elements diminishes with increase in the number of scan lines (i.e., number of pixels).
For this reason, the development of active matrix display devices has been going on at a brisk pace in recent years. Such display devices control the current flowing through the electro-optical element with an active element such as insulating gate field effect transistor (typically, thin film transistor or TFT) provided in the same pixel circuit as the electro-optical element. In an active matrix display device, the electro-optical elements maintain light emission over a frame interval. As a result, a large high-definition display device can be implemented with ease.
Incidentally, the I-V characteristic (current-voltage characteristic) of the organic EL element is typically known to deteriorate over time (so-called deterioration over time). In a pixel circuit using an N-channel TFT as a transistor adapted to current-drive the organic EL element (hereinafter written as “drive transistor”), the organic EL element is connected to the source of the drive transistor. Therefore, if the I-V characteristic of the organic EL element deteriorates over time, a gate-to-source voltage Vgs of the drive transistor changes, thus changing the light emission brightness of the same element.
This will be described more specifically below. The source potential of the drive transistor is determined by the operating point between the drive transistor and organic EL element. If the I-V characteristic of the organic EL element deteriorates, the operating point between the drive transistor and organic EL element will change. As a result, the same voltage applied to the gate of the drive transistor changes the source potential of the drive transistor. This changes the gate-to-source voltage Vgs of the drive transistor, thus changing the current level flowing through the drive transistor. Therefore, the current level flowing through the organic EL element also changes. As a result, the light emission brightness of the organic EL element changes.
In a pixel circuit using a polysilicon TFT, on the other hand, a threshold voltage Vth of the drive transistor or a mobility μ of a semiconductor thin film making up the channel of the drive transistor (hereinafter written as “mobility of the drive transistor”) changes over time or is different from one pixel to another due to the manufacturing process variation (the transistors have different characteristics), in addition to the deterioration of the I-V characteristic over time.
If the threshold voltage Vth or mobility μ of the drive transistor is different from one pixel to another, the current level flowing through the drive transistor varies from one pixel to another. Therefore, the same voltage applied to the gates of the drive transistors leads to a difference in light emission brightness of the organic EL element between the pixels, thus impairing the screen uniformity.
Therefore, the compensation and correction functions are provided in each of the pixels to ensure immunity to deterioration of the I-V characteristic of the organic EL element over time and variation in the threshold voltage Vth or mobility μ of the drive transistor over time, thus maintaining the light emission brightness of the organic EL element constant (refer, for example, to Japanese Patent Laid-Open No. 2006-133542 (hereinafter referred to as Patent Document 1)). The compensation function compensates for the variation in characteristic of the organic EL element. One of the correction functions corrects the variation in the threshold voltage Vth of the drive transistor (hereinafter written as “threshold correction”). Another correction function corrects the variation in the mobility μ of the drive transistor (hereinafter written as “mobility correction”).